In response to higher density, higher functionality, and higher speed of semiconductor integrated circuit devices of recent years, the following technique is known: circuits to be operated are selected by turning on/off current supply to functional circuits by means of a switch.
In a light-receiving amplifier circuit used for reading a signal of an optical disk, the optical power of laser is varied between recording and reproduction. Thus, when an optical signal is converted to an electrical signal and amplified, it is necessary to switch the gain resistances of an operational amplifier to set an output voltage within the output dynamic range of the amplifier circuit.
FIG. 12 is a circuit diagram showing a conventional light-receiving amplifier circuit. In this configuration, an operational amplifier having a gain resistance Rga and an operational amplifier having a gain resistance Rgb are connected in parallel, and the operational amplifiers to be operated are switched during recording and reproduction to perform gain control. The operational amplifiers are turned on/off by switching constant-current sources I9 and I10 or I11 and I12, which drive the operational amplifiers, by means of an external switch SW1.
However, the two operational amplifiers are connected in parallel and thus a circuit size increases and the wiring of the circuit is complicated.
Hence, a technique using an analog switch is proposed.
Referring to FIG. 13, the following will discuss JP10-107563A in which a light-receiving amplifier circuit uses an analog switch.
The output of a light-receiving element PD1 is connected to one input of an operational amplifier and simultaneously connected to two gain resistances. The gain resistance Rga is connected directly to the output of the operational amplifier and the other gain resistance Rgb is connected to the output of the operational amplifier via an analog switch ASW-15. During reproduction with low laser optical power, the analog switch ASW-15 is turned off, the gain resistance of the operational amplifier is set only with the gain resistance Rga, and high gain is obtained. Meanwhile, during recording with high laser optical power, the analog switch ASW-15 is turned on, the gain resistance of the operational amplifier is set with the parallel connection of the gain resistance Rga and the gain resistance Rgb, and the gain can be switched to low gain.
However, when turning on the analog switch, because of an on resistance changing with applied current, a parallel or series connection to the gain resistances results in a resistance value changed from the gain resistance having been set in response to an input signal, and an output voltage becomes different from a set value.
In view of this problem, an object of the present invention is to provide a differential amplifier circuit and an operational amplifier circuit which reduce the influence of an on resistance of analog switches, and a light-receiving amplifier circuit using the same.